Electrochemical cells are energy conversion devices. These devices are generally classified as a function of the direction of the energy conversion. Devices that produce chemical energy from electrical energy are referred to as electrolytic cells, whereas devices that produce electrical energy from chemical energy are referred to as fuel cells or batteries.
A fuel cell enables the production of electricity by means of two coupled chemical reactions: the oxidation of a reductive fuel on a first electrode, known as the anode, and the reduction of an oxidizing agent on a second electrode, known as the cathode. At the present time, hydrogen is commonly used as combustible and atmospheric oxygen is used as oxidizing agent.
A fuel cell finds particular usefulness in the field of transport, which, hitherto, has essentially used fossil energy mainly derived from petroleum. The use of this energy produces a large amount of carbon dioxide that contributes toward increasing the greenhouse effect on the planet. Other pollutants, such as particles or nitrogen oxides, are also produced by the use of petroleum-based fuels.
The main advantage of using a fuel cell using hydrogen and oxygen as feed gases is that the only product of the chemical oxidation and reduction reactions is water.
Among the various types of fuel cell that may be distinguished is the proton exchange membrane fuel cell, also known as a polymer electrolyte membrane fuel cell. Such a cell is formed from an elemental cell or a stack of elemental cells intercalated between a terminal plate forming the anode and a terminal plate forming the cathode.
FIG. 1 diagrammatically shows a proton exchange membrane fuel cell comprising a single elemental cell 1 intercalated between an anode 2 and a cathode 3. The elemental cell 1 comprises a polymer electrolyte membrane 4, known as the membrane 4, intercalated between two active layers 5a and 5b, for example of porous carbon. Each active layer 5a, 5b is in contact with a diffusion layer 6a or 6b, respectively, for example a paper or carbon fabric substrate. The diffusion layers 6a and 6b allow diffusion of the feed gases originating from delivery lines 7a and 7b to the active layers 5a and 5b, respectively. The delivery lines 7a and 7b are, for example, partly housed in bipolar plates 8a and 8b. In this figure, the bipolar plates 8a and 8b are directly in contact with the anode 2 and the cathode 3. Needless to say, in the case of a stack of elemental cells 1, a bipolar plate 8b of a first elemental cell comes into contact with a bipolar plate 8a of a second elemental cell, and so on.
In a proton exchange membrane fuel cell using hydrogen and oxygen as feed gases, the hydrogen is introduced in gaseous form at the anode 2, for example via the delivery line 7a, while the oxygen is introduced, also in gaseous form, at the cathode 3, for example via the delivery line 7b. In the presence of a catalyst, for instance platinum contained in the active layer 5a, the hydrogen releases electrons e− according to the following oxidation reaction:H2→2H++2e−
The electrons e− released into the active layer 5a join the active layer 5b via an electrical circuit 10 using the electrical energy produced by the fuel cell, and the protons H+, released during this first reaction, migrate to the active layer 5b by crossing the membrane 4. At the active layer 5b, the protons H+ combine with oxygen O2 and with the electrons e−, again in the presence of a catalyst, according to the following reduction reaction:2H++½O2+2e−→H2O
Overall, the following redox reaction takes place:H2+½O2→H2O
For a better energy yield, the oxidation and reduction reactions must take place within a certain temperature and pressure range. To ensure this adequate operating temperature, a heat-exchange fluid maintained at a temperature within this temperature range circulates in a pipe passing around or through the elemental cells 1.
The functioning of a fuel cell requires numerous fluid exchanges with devices peripheral to the fuel cell. In particular, the heat-exchange fluid requires passage through a device for maintaining its temperature. Similarly, the delivery lines 7a and 7b need to be connected to hydrogen and oxygen feed circuits. For the sake of reducing the bulk, the fluid delivery lines may open into the same component, known as the fluid delivery head or, more simply, the delivery head.
FIGS. 2a and 2b represent an example of a delivery head 21 in front view and cross-sectional view, respectively.
The delivery head 21 comprises an inlet connector 22a connected to the heat-exchange fluid pipe 23 at an inlet orifice 24a. The pipe 23 comprises a pipe portion 23a integrated into the delivery head 21. The pipe 23 extends inside or around the elemental cell(s) 1 and ends with a pipe portion 23b, which is, for example, integrated into the delivery head 21. This pipe portion 23b comprises an outlet orifice 24b that can receive an outlet connector, not shown. The delivery head 21 also comprises an inlet orifice 26a and an outlet orifice 26b for connecting the delivery line 7a to an external circuit such as a hydrogen feed circuit, and also an inlet orifice 28a and an outlet orifice 28b for connecting the delivery line 7b to an external circuit such as an oxygen feed circuit.
For correct functioning of the fuel cell, the membrane 4 must contain water in order to allow the transfer of protons H+ from the active layer 5a on the anode 2 side to the active layer 5b on the cathode 3 side. The membrane 4 is water-permeable. Consequently, a transfer of water takes place from the cathode 3 to the anode 2 via a diffusion mechanism due to the difference in water concentration on each side of the membrane 4. This water diffusion mechanism cohabits with a diffusion of other species such as nitrogen. Due to the temperature of the fuel cell, the water exiting the delivery line 7a is essentially present in gaseous form. In order to optimize the yield of the cell and to increase its service life, this water and the other species such as nitrogen are reinjected into the inlet of the delivery line 7a with the hydrogen. The circulation of hydrogen and the reinjection of water and nitrogen may be ensured by a circuit external to the fuel cell, comprising, for example, a pump or an ejector 30, shown in FIGS. 2a and 2b. The amount of water present in liquid form must, however, be precisely controlled. The reason for this is that an excessive amount of water in the elemental cell 1 prevents the feeding of hydrogen and oxygen to the active layers 5a and 5b, resulting in a voltage inversion at the terminals of the fuel cell and thus to electrolysis of water. In certain cases, for example in the case of a strong current demand, the fuel cell may be destroyed. In order to limit the risks of injection of water in liquid form at the inlet of the delivery line 7a, it is possible to place a conventional phase separator in the hydrogen feed circuit upstream of the delivery line 7a. However, such a phase separator is generally bulky. This bulk is an inconvenience for portable use. Moreover, a phase separator placed upstream of the delivery line 7a cannot control the amount of water present in liquid form within the elemental cell 1 due to the absence of control of the condensation of water in the pipe connecting the separator to the inlet orifice 26a. 